Reinsurance and Indemnification - Catastrophe reinsurance (Narrative) (Details) - Catastrophe Reinsurance Program $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) contract Contract reinstatement
Ceded Credit Risk [Line Items]
Catastrophe reinsurance loss amount	$ 3,500
Catastrophe reinsurance aggregate loss limit	7,900
Minimum
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	750
Maximum
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	1,000
Traditional Market
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	5,090
Catastrophe reinsurance loss limits per contract year	3,250
Exhausting loss from catastrophes	$ 4,250
Catastrophe reinsurance, co-participation percentage	5.00%
Number of reinstatements each year for each contract | reinstatement	1
Traditional Market, Placement One | Coverage - I
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 750
Exhausting loss from catastrophes	1,000
Catastrophe annual reinstatement of limits	$ 500
Contracts number | contract	0.3333
Traditional Market, Placement Two | Coverage - II
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 236
Number of reinstatements each year for each contract | reinstatement	1
Contracts number | Contract	2
Term of catastrophe reinsurance agreement contract (in years)	8 years
Percentage of co-participation	5.00%
Traditional Market, Placement Three | Coverage - III
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 1,610
Contracts number | Contract	6
Term of catastrophe reinsurance agreement contract (in years)	1 year
Insurance Linked Securities (ILS)
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 1,950
Insurance Linked Securities (ILS) | Six Contracts
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 1,300
Contracts number | contract	6
Insurance Linked Securities (ILS) | Two Contracts
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 325
Contracts number | contract	2
Insurance Linked Securities (ILS) | Coverage - III
Ceded Credit Risk [Line Items]
Number of reinstatements each year for each contract | reinstatement	1
Contracts number | Contract	2